This invention relates to a linear translation subsystem of an image-forming apparatus of the lathe bed scanning type for creating an image on sheet media held on a rotating imaging drum, and more particularly to the alignment of the translation bearing rod or rods to the imaging drum surface.
Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a highspeed, high-volume printing press to produce an example single of an intended image. These intended images may require several corrections and be reproduced several times to satisfy customers requirements, which results in loss of profits. By utilizing pre-press color proofing, time and money can be saved.
One such commercially available image-forming apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708, is an image forming apparatus having half-tone color proofing capabilities. This image forming apparatus is arranged to form an intended image on a sheet of thermal print media by transferring colorant from a sheet of colorant donor material to the thermal print media by applying a sufficient amount of thermal energy to the colorant donor material to form an intended image. This image forming apparatus is comprised generally of a material supply assembly or carousel; lathe bed scanning subsystem, which includes a lathe bed scanning support frame, translation drive, translation stage member, printhead, and imaging drum; and thermal print media and colorant donor material exit transports.
The operation of the image-forming apparatus includes metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length and transported to the imaging drum, registered, wrapped around, and secured onto the imaging drum. Next, a length of colorant donor material (in roll form) is also metered out of the material supply assembly or carousel, then measured and cut into sheet form of the required length. It is then transported to and wrapped around the imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the imaging drum). A rotatable vacuum imaging drum is preferred herein.
After the colorant donor material is secured to the periphery of the imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the colorant donor material on the spinning vacuum imaging drum while it is rotated past the printhead that will expose the thermal print media. The translation drive then traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the colorant donor material is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The colorant donor material is then transported out of the image forming apparatus by the colorant donor material exit transport. Additional colorant donor materials are sequentially superposed with the thermal print media on the vacuum imaging drum. Then they are imaged onto the thermal print media as previously mentioned, until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image forming apparatus by the receiver sheet material exit transport.
The material supply assembly comprises a carousel assembly mounted for rotation about its horizontal axis on bearings at the upper ends of vertical supports. The carousel comprises a vertical circular plate having in this case six (but not limited to six) material support spindles. These support spindles are arranged to carry one roll of thermal print media, and four rolls of colorant donor material to provide the four primary colors used in the writing process to form the intended image, and one roll as a spare or for a specialty color colorant donor material (if so desired). Each spindle has a feeder assembly to withdraw the thermal print media or colorant donor material from the spindles, which is to be cut into a sheet form. The carousel is rotated about its axis into the desired position, so that the thermal print media or colorant donor material (in roll form) can be withdrawn, measured, and cut into sheet form of the required length, and then transported to the vacuum imaging drum.
The scanning subsystem or write engine of the lathe bed scanning type comprises the mechanism that provides the mechanical actuators, for the vacuum imaging drum positioning and motion control to facilitate placement, loading onto, and removal of the thermal print media and the colorant donor material from the vacuum imaging drum. The scanning subsystem or write engine provides the scanning function by retaining the thermal print media and colorant donor material on the rotating vacuum imaging drum, which generates a once per revolution timing signal to the data path electronics as a clock signal while the translation drive traverses the translation stage member and printhead axially along the vacuum imaging drum in a coordinated motion with the vacuum imaging drum rotating past the printhead. This is done, with positional accuracy maintained, to allow precise control of the placement of each pixel in order to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the linear translation subsystem, printhead, and the imaging drum and its rotational drive. The translation stage member and printhead are supported by two translation bearing rods that are ideally straight along their longitudinal axis. This permits low friction movement of the translation stage member and the translation drive. The translation bearing rods are positioned and supported at their ends by bores in the outside walls of the lathe bed scanning support frame or write engine.
The two translation bearing rods are arranged between the translation stage member and the printhead. A front translation bearing rod is arranged to locate the axis of the printhead precisely on the axis of the imaging drum with the axis of the printhead located perpendicular, vertical, and horizontal to the axis of the imaging drum. The translation stage member front bearing is arranged to form an inverted xe2x80x9cVxe2x80x9d and provides only that constraint to the translation stage member. The translation stage member is held in place by its own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod. This is done so that no over constraint of the translation stage member causes it to bind, chatter, or otherwise impart undesirable vibration to the translation drive or printhead during the writing process. Such vibrations can cause unacceptable artifacts in the intended image. The rear bearing enables this advantage by engaging the rear translation bearing rod only on diametrically opposite side of the translation bearing rod on a line perpendicular to a line connecting the centerlines of the front and rear translation bearing rods.
Although currently available image forming apparatus are satisfactory, they do have certain drawbacks. First, alignment of the linear translation subsystem limits the output quality that the intended image can be exposed onto the thermal print media within the intended image, intended image to intended image within a given image forming apparatus, or intended image to intended image, from one image forming apparatus to another image forming apparatus. More importantly, the same is true of the alignment of the printhead to the imaging drum surface or the thermal print media and colorant donor material. With existing image forming apparatus, alignment of the linear translation subsystem, and the printhead relative to the imaging drum surface or the thermal print media and colorant donor material, is limited by the constraints imposed by the accuracy of currently available manufacturing technology to produce the lathe bed scanning engine.
The present invention has several advantages. First, the present invention provides an increase in image quality of the intended image, intended image to intended image, and the intended image from image forming apparatus to image forming apparatus. Second, the need to automatically focus the printhead is reduced or eliminated by improved alignment of the linear translation subsystem and printhead to the imaging drum surface, and also the thermal print media, and colorant donor material. Third, the linear translation subsystem is aligned, as is the printhead to the imaging drum surface, thermal print media and colorant donor material. This considerably reduces required maintenance, costs, and complexity of the image forming apparatus. Finally, the present invention provides an added margin for depth of focus, and for handling a larger range of media thickness tolerance.
In general, currently available image forming apparatus have fixed translation bearing rods. The linear translation subsystem is therefore fixed and relies upon the accuracy of currently available manufacturing tolerances. Among other things, the present invention reduces or eliminates reliance upon manufacturing tolerances by employing adjustable and lockable translation bearing rods.
According to one aspect of the present invention, this invention is an image forming apparatus for writing images to a thermal print media, comprising:
a) a rotatable imaging drum comprising a drum housing;
b) a motor capable of rotating the imaging drum;
c) print media mounted on the imaging drum;
d) a printhead adapted for forming the intended image, the printhead being movable along a line parallel to a longitudinal axis of said imaging drum as the imaging drum rotates; and
e) a linear translation subsystem for moving the printhead, the linear translation subsystem including one or two adjustable translation bearing rods, which are parallel to the longitudinal axis of the imaging drum. The printhead is supported on the translation bearing rod, and the translation bearing rod is adjustable for minimizing misalignment error between the linear translation subsystem and the imaging drum.
The present invention also includes a process for minimizing misalignment of a linear translation subsystem of an image forming apparatus, comprising the following steps:
a) temporarily replacing a printhead with a capacitance probe mounted in a probe holder;
b) mounting sheet print media and colorant donor material on an imaging drum;
c) rotating the imaging drum at writing speed;
d) allowing a linear translation subsystem to move a translation stage along the imaging drum;
e) using the capacitance probe to measure a distance, or gap, between the capacitance probe and the surface of the imaging drum;
f) adjusting any misalignment of the linear translation subsystem to the imaging drum surface;
g) locking the translation bearing rods in place;
h) removing the capacitance probe; and
i) reinstalling the printhead.